MEMS (microelectromechanical structures) are flexible cantilever-type structures that have many applications and an even greater potential in today's advancing technologies. MEMS are formed using semiconductor processing technology and over substrates that may be formed of semiconductive or insulating materials. For example, MEMS may be deflectable mirror structures that can reflect light in different directions. In recent years, the projection-display industry has undergone a period of explosive growth. Until several years ago, such projection display systems were predominantly based on ether cathode-ray tube (CRT) or active-matrix liquid crystal display (LCD) technology. All of these traditional display systems, however, suffer from limitations that compromise their performance or the spectrum of their applicability. LCD- and CRT-based systems are limited in their ability to support high-brightness applications, and they suffer from uniformity and stability problems in large-scale applications.
An emerging projection display technology called Digital Light Processing (DLP) accepts digital video and transmits to the eye a burst of digital light pulses that the eye interprets as a color analog image. Digital light processing is based on a MEMS device known as the Digital Micromirror Device (DMD) invented in 1987 at Texas Instruments Inc. The DMD is a fast reflective digital light switch, which combines with image processing, memory, and a light source and optics to form a digital light processing system. The DMD is a light switch that uses a plurality of electrostatically controlled MEMS mirror structures to digitally modulate light, producing high-quality imagery on screen.
The MEMS used to form the plurality of light switches are typically formed over CMOS memory devices and using CMOS-like processes. Each light switch includes a deflectable aluminum alloy mirror that can reflect light in different directions depending on the state of the underlying memory cell. Mirror quality has been found to be the most important factor in the performance of DLP and similar systems. The deflectable mirror film of the MEMS, commonly an aluminum alloy and preferably an aluminum-silicon-copper material, is formed over a releasable layer commonly referred to as a sustain layer. The sustain layer is formed between the mirror film and the substrate and is removed after the mirror layer has been formed into a plurality of discrete mirror structures that are anchored to the substrate to form cantilever-type MEMS. A preferred sustain layer material is amorphous silicon. When amorphous silicon is used as the sustain layer, however, spiking problems between the metallic mirror layer and the amorphous silicon were prevalent. To avoid this, an insulating barrier layer was added both above the metallic mirror layer and below the metallic mirror layer, i.e., between the metallic mirror layer and the release layer. Even with insulating barrier layers formed above and below the metallic mirror layer, spiking still occurred because, after etching to form the discrete mirror structures, exposed sidewalls of the metallic mirror layer were present. Sidewall spiking causes mirrors to be coupled to one another such that they could not operate independently and also resulted in spiking between the mirror layer and the release layer, resulting in other functional problems. It is now known to use oxide spacers on the sidewalls to address this problem. The formation of sidewall oxide spacers on a MEMS structure, however, is a process that is difficult to control.
Oxide spacers are formed by first forming a discrete, etched mirror structure then forming an oxide layer over the structure, including vertically over the sidewalls. A blanket etch operation is then carried out to remove horizontal portions of the oxide layer while desirably leaving vertical sections intact to form sidewall spacers. The blanket, “etch back” process is difficult to control and underetching produces a residual oxide film linking mirrors together after the sustain layer is released. If the mirrors are coupled to one another in this manner, they cannot operate independently. Overetching the oxide film attacks the oxide layer above the mirror layer. When the upper oxide layer is attacked, its thickness is reduced. Attack of the upper oxide layer significantly influences the bending characteristics and reflectivity of the mirrors which are very sensitive to the thickness and uniformity of the oxide layers above the metallic mirror layer. Since the horizontal and vertical portions of the oxide film are the same material, in fact, the same film, there is no selectivity between the vertical and horizontal portions and the blanket etch process cannot be reliably terminated when only the horizontal sections are removed. Ironically, using conventional methods and substructures, the layer upon which the spacer etch is desirably terminated is also an oxide layer—the critical upper oxide layer disposed beneath the spacer oxide film being etched. Since the materials are the same, it is impossible to automatically endpoint the etch process which often results in the subjacent “upper oxide barrier layer” being attacked, damaging the mirror structure.
It would therefore be desirable to provide a method and structure for producing a MEMS mirror structure by reliably etching an oxide layer formed over the mirror structure and accurately terminating the etch operation so that no oxide remains between mirrors, spacers are reliably formed, and the oxide formed over the mirrors is free from damage.